Gas detection and measurement by means of sensors is well known in the art. There are many types of sensors available designed for various applications. One of the most common types of sensors is the electrochemical gas sensor. Such sensors are widely available and described in numerous United States patents, including U.S. Pat. Nos. 3,767,552, 3,824,168, 3,909,386, 3,992,267, 4,132,616 and 4,324,632. Electrochemical sensors are widely used for measuring oxygen concentrations and the concentration of toxic gas detection for work-place safety, emission monitoring and control of pollutants.
Most electrochemical sensors are designed such that a gas of interest is brought into reactive contact with the electrodes such that any target analyte in the gas is oxidized or reduced at a sensing electrode within the sensor. The gas typically passes through one or more diffusion barriers, such as a porous membrane, capillary sintered disk, etc. to reach the electrode. The majority of sensors are operated such that the response current is limited by the rate at which the gas can diffuse into the sensor. A diffusion limited sensor has the advantageous properties of a linear response to the gas concentration, stable output with small changes in operating potential due to environmental or instrumental changes (e.g. variations in power supply voltage), and either a small or at least well defined variation in output with temperature and pressure.
In a typical design, a sensor will contain two or more electrodes within the sensor in contact with an electrolyte. One electrode is designated the sensing electrode. The gas of interest enters the sensor and moves by diffusion through the membrane and any other diffusion barriers in the gas path to the electrode. The gas is consumed at the sensing electrode in either an oxidation or a reduction process, and the resulting electrical charge passes from the electrode, through the external circuit to the counter electrode. The magnitude of this electric current generates an output signal. At the counter electrode, which must also be in contact with the electrolyte, an equal and opposite electrochemical reaction occurs.
The magnitude of the output signal is determined by the concentration of target analyte in the gas, and by the diffusivity of the gas path through which the gas must pass to reach the sensing electrode. The diffusivity is defined here as a measure of how much gas at unit concentration will diffuse into the sensor per second. If the diffusivity of the sensor is known, then the gas concentration can be calculated by measuring the output current from the sensor.
In the operation of electrochemical sensors, it is customary to periodically calibrate the zero and span performance of the sensor in order to re-confirm or re-establish the measurement accuracy. Calibration of the sensor elements is important in achieving accurate measurement with the sensor because sensors and associated electronics tend to suffer from zero drift over time in either a positive or negative direction. In accordance with conventional practice, the zero drift of the sensor element over time is compensated for or adjusted by periodic zero calibrations utilizing a known zero calibration gas composition which contains none of the target gas. A similar problem exists with respect to the sensor and associated electronics span signal which gradually will change over a period of time due to aging of the sensor elements. This may similarly be corrected for or adjusted by periodic span calibrations of the sensor circuitry utilizing a calibration gas which contains a known quantity of test gas.
Calibration of the instrument can be a substantial operational problem as it requires the purchase, storage and use of two different calibration gasses (one with zero target analyte for obtaining a zero calibration reading and the other with a known concentration of target analyte for obtaining a span calibration reading) and the instrument is normally taken off-line for calibration. In many situations, such as in the case of industrial controls, the removal of the sensor from the control process may result in the loss of production time and considerable expense while the process is shut down until the control instrumentation can be calibrated and returned. In the alternative, to avoid shut-down of the process, substitute instruments can be used to replace instruments undergoing calibration. While generally effective for avoiding shut-down, this method requires a larger inventory of instruments.
Methods and apparatus to permit automatic in-line calibrations have been developed, such as those described in U.S. Pat. Nos. 4,116,612, 4,151,738, 4,322,964, 4,384,925, 4,489,590 and 5,239,492. While generally effective, such devices substantially add to the cost of the instrument. In addition, such instruments still rely on the presence of test gases of known concentration (i.e., a gas containing no target analyte and a gas containing a known concentration of target analyte). For oxygen sensors, ambient air is often used as one of the test gases as the concentration of oxygen in well ventilated areas is a constant 20.9 volume percent. However, a source of oxygen free gas is still necessary to fully calibrate an oxygen sensor.
U.S. Pat. No. 4,829,809 describes a calibration method which does not require a known test gas concentration. The method passes a test gas of unknown concentration over the sensor through a calibration flow system of known volume. After flushing the system, the flow of test gas is stopped and the calibration system is sealed. The output current decays to zero as the sensor consumes target analyte in the gas. Using Faraday's Law, the sensitivity of the initial gas concentration can be found. While generally effective, the process is slow as substantial time is required for the current to decay exponentially to zero, and the process and prone to errors. A similar system is described in U.S. Pat. No. 4,833,909, but is subject to the same problems.
Other approaches have focused on the electrical properties of the sensor. For example, U.S. Pat. Nos. 5,202,637 and 5,611,909 apply a small potential to the normally constant potential between the sensing electrode and the counter electrode and monitor the electrical current response of the sensor. While providing a simple in-situ test that an instrument or controller can automatically perform on the sensor, this method will only detect those modes of sensor failure which affect the electrical properties of the working electrode, such as loss of volume due to dry-out from an aqueous based electrolyte. The test is unable to detect sensor faults resulting from other problems.
Accordingly, a need exists for an inexpensive system and method for quickly, simply, and reliably calibrating an electrochemical sensor.